A low noise amplifier (LNA) is commonly the first amplifying stage in a radio frequency receiver. The LNA is typically arranged to amplify a received signal to an amplitude suitable for further processing and decoding. To illustrate typical uses of an LNA two examples will be described with reference to FIGS. 1 and 9.
As illustrated in the simplified communications system 100 of FIG. 1, an LNA 130 receives a potentially weak signal captured by an antenna 110 and amplifies it for subsequent use in a receiver system 140. In some implementations an LNA may receive a signal directly from an antenna; in other implementations a front-end module (FEM) 120 may be provided. An FEM may comprise, amongst others, one or more filters, switch elements, duplexers and diplexers. Upstream components are those to the left of the LNA in FIG. 1 (e.g. FEM 120) and downstream components are those to the right of the LNA in FIG. 1 (e.g. receiver 140).
FIG. 9 illustrates a schematic diagram of an exemplary Direct Conversion Receiver (DCR) as known in the art. The DCR makes use of a LNA to amplify a received signal. In the example of FIG. 9, a wide range of radio communication signals are received at antenna 900, which is tuned by a wide bandwidth pre-selection filter 902. The filtering behaviour of pre-selection filter 902 may arise out of the physical and electromagnetic characteristics of the antenna design, perhaps due to optimisation for the frequency band of the desired telecommunication application. Pre-selection filter 902 may also include one or more tuned circuits, which are used to remove frequency components of the input that are far outside the intended range of reception frequencies. The received signal is typically of very small amplitude, and requires amplification by LNA 904 before further processing can be performed. In this example, LNA 904 must operate at high frequencies (at least the transmission frequency of the signal the receiver is intended to receive), commonly referred to as radio frequency, and also provide very low noise insertion due to the weakness of the incoming signal.
In order to select the appropriate signal from the many signals received at antenna 900, the received input must be filtered. However, the high selectivity of the filter profile that would be required to isolate one signal at radio frequency makes filtering at this stage either unrealistic (given the manufacturing tolerances of commonly available components) or undesirably expensive. Hence, before signal selection can occur, the frequency of the desired signal must be down-converted by mixing the input signal with a signal generated by local oscillator 906. A direct conversion receiver converts the desired signal directly to baseband frequency by mixing it with a local oscillator signal of the same frequency as the carrier frequency of the desired signal. This has the effect of translating the desired signal to be centred on zero frequency.
In order to extract both the I and Q components, the input signal must be mixed with both in-phase and quadrature shifted versions of the local oscillator signal, which are generated by quadrature generator 908. The exact phase of the received signal cannot be predicted due to the unknown phase shift that will occur during transmission. Hence, the local oscillator must synchronise with the received signal in order to ensure the necessary phase relationship. This synchronisation may be achieved by establishing a phase reference, for example by using a phase locked loop (PLL) or by rotating the signal after down-conversion by digital means. The input signal is mixed with the in-phase local oscillator signal by mixer 910, and with the quadrature phase local oscillator signal by mixer 912. Mixers 910 and 912 perform multiplication between the input signal and the appropriate local oscillator signal in order to achieve the required frequency down-conversion.
The desired I and Q components can then be isolated using low pass filters 914 and 916 respectively, which are used to suppress unwanted frequencies associated with signals adjacent in adjacent channels etc. Finally, analogue to digital converters (A/Ds) 918 and 920 convert the I and Q components into binary representations of the I and Q message data 922 and 924. Once in the digital domain, further processing can be performed on the I and Q data, including recombination of the components to form the original data message. The original data message can then be used by the receiving device. As with FIG. 1, upstream components are those to the left of the LNA in FIG. 9 (e.g. pre-selection filter 902) and downstream components are those to the right of the LNA in FIG. 9 (e.g. mixers 910 and 912).
In certain implementations of the DCR of FIG. 9, components 904 to 924 form part of a Radio Frequency Integrated Circuit (RFIC) 950. In these cases, the design of the LNA must be suitable for RFIC manufacturing and preferably take up a minimal silicon die area.
As demonstrated by the previous examples, most modern communications systems comprise a plurality of processing components for receiving signals. Each processing component will contribute to the degradation of a signal-to-noise (SNR) ratio of the signal received from the antenna. Friis' equation provides a formula for calculating a total noise factor for a communications system:Ftotal=F1+(F2−1)/G1+(F3−1)/G1G2+ . . . +(Fn−1)/G1G2 . . . Gn-1                 wherein Fn and Gn are respectively the noise factor and power gain of an nth component in a cascade of input stages, the noise factor being the ratio of the SNR into a component and the SNR out of a component. Reference may be also made to a noise figure, which is a noise factor expressed in decibels. If the LNA is the first amplifying stage of the communications system, then according to Friis' equation, the LNA sets the minimum noise factor of the system, i.e. F1=FLNA. Hence, it is important that the LNA has a low noise factor, i.e. that the amount of noise introduced by the LNA is minimised.        
The noise factor of a LNA can also affect the design of large-scale telecommunications systems. For example, a telecommunications system may comprise a number of electronic devices that communicate with a fixed network using wireless communications. As the LNA typically sets the minimum noise factor of a receiving device in a telecommunications system, the noise factor of the LNA influences the sensitivity of these electronic devices to wireless signals. If the noise factor is high, the sensitivity decreases accordingly. This shortens the range of an electronic device thus making the design of the telecommunications network more challenging and more expensive. For example, the noise factor of an LNA implemented in a number of electronic devices can influence the number of base stations that are needed; more base stations being needed if the noise factor is high.
Radio frequency receivers can be configured to operate within a number of different radio frequency bands. For example a receiver for a mobile station (or cellular telephony device) can be configured to operate within any of the following bands: Global System for Mobile Communications (GSM), 850, 900, 1800, and/or 1900, Wideband Code Division Multiple Access (WCDMA), High Speed Packet Access (HSPA) and/or Long Term Evolution (LTE) Bands 1, 2, 3, etc. This allows a mobile station containing such a receiver to be used in different areas where varying subsets of the above radio frequency bands are supported (e.g. to enable roaming). However, this requires processing stages capable of operating across a wide band of frequencies. In particular, this either requires multiple low-cost LNAs for amplifying a plurality of frequency bands or wideband LNAs.
There is thus a need in the art for an LNA with a low noise factor. For implementation in mobile devices an LNA should have a small overall size. An LNA should also have a low cost, low current consumption and be suitable for high volume manufacturing. Hence, in order to design a competitive LNA, there are typically several figures-of-merit to be fulfilled, wherein several of the requirements for an LNA are difficult to achieve simultaneously.